The structural reliability of a fast breeder reactor cover was evaluated using probabilistic fracture mechanics. The effects of important factors (initial crack distribution, crack detection level, crack growth rate, incidence of events such as earthquakes, in-service inspection for cracks, weld toughness, residual stresses, and environmental effects) on the reliability of structures were estimated, and methods for maintaining proper reliability were studied. It was found that the probability of the reactor losing its function is∼10−8in the terminal stage of the plant lifetime, even when the most conservative initial crack distribution and crack detection level are assumed. However, the prerequisite of such a condition is the use of appropriate leak monitors or proper atmospheric control. It was also confirmed that the degree of improvement in reliability through in-service crack detection, which was considered to be important, is not necessarily significant for the reactor cover.